The electronics industry continues to strive for powerful, highly functional circuits. Significant achievements in this regard have been realized through the fabrication of very large-scale integrated circuits on small areas of silicon wafers. Integrated circuits of this type are manufactured through a series of steps carried out in a particular order. The main objective in manufacturing such devices is to obtain a device which conforms to geographical features of a particular design for the device. To obtain this objective, steps in the manufacturing process are closely controlled to ensure that rigid requirements, for example, of exacting tolerances, quality materials, and clean environment, are realized.
Dynamic random access memories (DRAMs) are a widely used memory type. They are oftentimes preferred over other memory types because they can be implemented to provide an extraordinary number of memory cells in a relatively small area. The vast majority of DRAMs are implemented using MOS (metal oxide semiconductor) technology, which refers to any integrated circuitry in which n-channel and/or p-channel field effect transistors are used.
MOS devices are fabricated from various materials, including electrically conductive, electrically nonconductive and electrically semi-conductive materials. A frequently-used semi-conductive material is silicon. Silicon can be made conductive by doping (i.e., introducing an impurity into the silicon crystal structure) with either an element such as boron or with an element such as phosphorus or arsenic. In the case of boron doping, electron holes become the charge carriers and doped silicon is referred to as positive or p-type silicon. In the case of phosphorus or arsenic doping, the additional electrons become the charge carriers and the doped silicon is referred to as negative or n-type silicon.
Silicon is used either in single-crystal or polycrystalline form. Polycrystalline silicon is referred to herein as "polysilicon" or as "poly." Due to its effectiveness as a conductor, polysilicon has been used in place of metal for many types of MOS applications. However, the higher connectivity characteristic of metal has motivated a number of semiconductor manufacturers to use a layer of refractory silicide on transistor gates to increase the device's speed.
Device speed, sometimes referred to as access time, has drawn significant attention in recent years. Particular attention has been given to increasing device speed while using ever-decreasing package sizes. For many DRAM applications, these efforts have been directed to using fewer elements to implement each memory cell in the DRAM array. For example, by using fewer transistors to implement each memory cell, the overall semiconductor real estate used per cell area can be decreased. A successful example of using one transistor to implement each memory cell is disclosed in the above-referenced U.S. patent application, entitled "Data Storage Circuit Using Shared Bit Line and Method Therefor."
Even with a reduced number of elements used to implement such memory cells, there continues to be a need to reduce the layout area as well as the integration area required for each cell. For example, memory cells of this type typically include a capacitor or other device designed to hold a charge representing a data bit. The size of the memory cell is partly dependent on the size of this charge-holding device. To sufficiently maintain the charge over an extended period of time and over a variety of environmental conditions, the charge-holding device is typically designed to be one of the larger elements in the memory cell. Such large devices have been the subject of these efforts to reduce the layout and integration used in designing such cells.